The present invention relates to an apparatus for applying a coating liquid on the surface of a medium already printed and more particularly, a printed sheet coating apparatus, which prevents a coating liquid from scattering outside a printed sheet, and which can correctly control a coating area.
Conventionally, printers based on various principles have been put into practical use as simple and easy means for forming a printed image on various printing media such as a paper sheet, a plastic sheet and cloth. Especially, there have been well known and widely accepted printers in practical aspects, which can print a color image and in addition form that of large size with simplicity and ease, which are: an ink jet printer using a liquid ink, a solid-ink ink jet printer using a hot melt ink, a thermal printer using a wax-ink ribbon and a thermal printer using a sublimation dye ribbon and a color electrostatic printer using a liquid developer.
While these printers can produce a printed image with simplicity and speediness, there have been a fault of poverty in various kinds of resistance in the image. That is, one has no water resistance of its colored material in an image and in addition no water resistance of its coated recording layer which is formed on a recording medium. Another one has a small resistance to surface injury on its image and thereby the image is subject to scratches. A still another one is that a colored material is chemically unstable and thereby the color thereof is faded away by a finger print. In general, printed images obtained by the printers have had poor light resistance and has been easy to fade away.
In order to complement such faults, there has been proposed a technique in which an image is protected by a transparent laminated film adhering to the image surface. However, since a laminated film is expensive and the image surface is covered with a thick plastic sheet, a feeling of the surface is conspicuously changed. Moreover, an expensive, large scaled apparatus is required for lamination and it demands an expertise in correctly performing the lamination in the case of a large image.
There has been proposed another technique in, for example Publication of Jpn. Pat. Appln. KOKAI Publication No. 62-101482, in which a transparent hot melt coating is applied on the surface of a printed image. This method has a smaller change in feeling of the print surface than the lamination. However, there is a need for a special ribbon and an special apparatus for the coating, which causes high cost, and besides, there is a restriction that the surface which receives the coating must have a good flatness.
As means, which does not accompany such faults or restrictions as described above, there has been presented a technique in U.S. Pat. No. 4,724,025 assigned to the same applicant as that of the present application in which spray coating is applied on a printed image. According to the technique, coating in use for various objects can be achieved by selection of a coating liquid and besides not only does no problem happen even if a surface has a concavity and convexity profile thereon, but also a change in feeling of a printed surface can be suppressed to a minimum level. A structure of a printing apparatus in which a coating apparatus shown in the above mentioned U.S. Pat. No. 4,724,025 is incorporated is shown in FIG. 17 and the structure will be described.
In FIG. 17, a scanning drum 206 holds a print sheet on its outer circumferential surface in a winding manner. A reference mark 203 indicates an ink jet print head, reference marks 204, 205 indicate a guide rail and a feed screw for secondary scanning. A reference mark 217 indicates a spray device, which comprises a spray gun 218 and a coating liquid tank 219, and which moves on guide rails 220. Printing is performed by rotation of the drum 206 and the coating liquid is applied by being jet with the spray gun 218 on the printed medium which has been finished with printing. As coating means, a spray gun with which a coating liquid is sprayed together with compressed air, a device with which a mist is produced by an ultrasonic vibrator or the like is used.
However, with the coating apparatus employing a spray head according to the conventional techniques, which is used for coating on a printed sheet, there has been a fault that a coating liquid is scattered beyond the periphery and thereby the surroundings are greatly contaminated since a flying direction of the coating liquid is not fixed. Therefore, while there is no problem when the small number of printed media are treated by hand coating or when coating is carried out in a spray booth equipped with a suction duct of exclusive use, the apparatus cannot be used in an office room or in a place indoors where no special facilities are provided and in these cases loss of the coating liquid have been tremendous.
While in the cases where a conventional lamination film or a hot melt coating material is subjected to transfer coating, coating can be achieved at a constant thickness all the time regardless of a feed rate of a printed medium, in the case of a spray coating method a thickness of coating has been fluctuated in dependence on a change in the feed rate of the printed medium.
In a conventional spray coating method, since a boundary of a spraying area is vague, some of a coating liquid which flies outside the boundary contaminates transportation means and the like attaching thereto if coating is conducted so as to cover a boundary area, which has been an obstacle for realization of a coating apparatus for a printed sheet.
Further, in the case where lamination or coating is conducted on a printed sheet different in width by a conventional technique, changes in specification of necessary materials and setting greatly different operational conditions have been required and thereby surface protection freely corresponding to different widths of various media has not been realized with ease.
Moreover, according to a conventional technique, in the cases where a kind of lamination or coating is changed, a preceding large scale preparation is required and thereby a change in kind of lamination or the like with a small amount as a unit has not been able to be conducted with ease.